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THEIR COMPOS1TI 



in their aggregation; into the character of its fossils, which 

 thus throw light on the geographical conditions of the 

 period ; and, finally, into its industrial value, as bearing on 

 the wants and progress of civilisation. Adopting this me- 

 thod, we find the Carboniferous system composed in the 

 main of sandstones, shales, fire-clays, ironstones, limestones, 

 and coals, all many times alternating with each other, and 

 in some districts attaining to a thickness of 12,000 or 

 14,000 feet. Of course, during the deposition of such a 

 vast thickness of strata, and which necessarily implies the 

 lapse of long ages, there must have been frequent changes 

 in the relative levels of sea and land ; and hence some of 

 these sediments were laid down in deep and others in shal- 

 low water, while the shallower beds were once more sunk 

 to greater depths, and overlaid by newer sediments. In 

 this way the Carboniferous system consists, in most regions, 

 of several series of strata, and in the British Islands these 

 are generally arranged and named as follows : 



1. Upper or true coal-measures. 



2. Millstone grit or sandstone series. 



3. Carboniferous or mountain limestone. 



4. Lower coal-measures or carboniferous shales. 



Although these several series have evidently been depo- 

 sited in waters of various depths and under somewhat dif- 

 ferent geographical conditions the lower being apparently 

 more estuarine, the mountain limestone being more marine, 

 the millstone grit more littoral, and the upper more terres- 

 trial still there is a great family resemblance, so to speak, 

 between them, and, with the exception of the coal-seams, 

 they are all strictly sedimentary, and bear in their structure 

 and texture abundant evidence of the aqueous agencies con- 

 cerned in their formation. In the sandstones and grits 

 often ripple-marked, rain-pitted, and worm-burrowed we 



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